Modulation system



y 1941- K. SCHLESINGER' 5 MODULATION SYSTEM Filed Dec. 25, 195'? I lnkenzon- 0 I 1 Z] 3 WWW Patented July 8, 1941 MODULATION SYSTEM Kurt Schlesinger, Berlin, Germany, assignor, by

mesne assignments, to Loewe Radio, Inc., a corporation of New York Application December 23, 1937, Serial No. 181,320 In Germany December 24, 1936 1 Claim.

The present invention relates to the elimination of disturbances in television amplifier and transmitter systems due to potential drop in the anode potential supply lines on the transmitter stage.

In all carrier frequency amplifiers, and more particularly in the amplifying stages of wireless television transmitters, so far as they act on the modulated carrier wave of the image, there is to be observed, if these amplifiers are fed by condenser storage means, an effect which will be referred to as signal weakening or, briefly, as fatigue. This effect consists in the following:

In the transmission intervals the transmitter sends merely synchronisation signals with approximately of full load. The plate potential storage means is then fully charged and I the plate potential is very high. The plate potential rectifier operates idly. If now powerful white signals are suddenly transmitted, these signals require full amplifier and carrier energy. The plate potential requirements of the transmitter increases and the plate potential storage means is discharged. If the signals persist at full strength for a comparatively long time, the plate potential drops, the signal energy diminishes and the transmitter becomes fatigued. This state of fatigue occurs all the more quickly the smaller the storage capacity and the greater the plate current requirement of the transmission tubes may be. The reverse case occurs when the transmitter again changes from full load to idle operation (white to black). The condensers are then run down, the black energy of the transmitter is below normal; the charge of the condensers increases, however, owing to the now smaller current requirement of the transmitter, so that the correct energy which corresponds to the black value again returns corresponding to the charging time constant of the anode storage to the normal initial value.

The invention will be clearly understood by Way of exemplary embodiments of the fundamental idea of the present invention shown in the accompanying drawing, wherein,

Figures 1, la and 1?) show time diagrams of the course of potential of transmission signals by which the fundamental idea is explained.

Figure 2 shows exemplary circuit arrangement for performing the invention.

Fig. 1 shows without consideration to scale the theoretic course of the control potential at the modulator input. Between the times 0 and 1 the control potential should be assumed at the synchronisation value corresponding to a black value of the contents of the image. At the point I the control potential changes to a value corresponding to white in the image (full load) and after a time T synchronisation is again effected between 2 and 3.

Fig. 1a shows the true course of the effective aerial current of the signals shown in Fig. 1. After cessation of the full load it again increases to the normal value for synchronisation signals 0. In television operation the control periods T amount at the most to second for simple scanning or second for interlaced line scanning, or more generally to the simple or double length of period of the mains, with which the interlaced line scanning always requires to run synchronously.

Fig. 1b characterises the method of correction by distortion of the signal course.

The invention describes method of overcoming this signal fatigue. There are in substance three methods:

(1) Load compensation of the current source,

(2) Use of screening grid tubes in the transmitter,

(3) Correcting methods for the modulating potential.

The present invention relates to the latter method of influencing the modulation potential. Such a correction of the modulating potential in accordance vn'th the invention, has been employed with good results by the applicant in the case of television transmitters using three electrode tubes, and fundamentally may also be extended to transmitters of high output. The basic idea of the method is illustrated in Fig. 1b. It consists in so distorting the control potential, which is supplied by the modulation amplifier to the transmitter, in relation to the ideal form shown in Fig. 1 that the fatigue effect of the transmitter is just compensated and the aerial current itself finally has imparted thereto an ideal rectangular form. In Fig. 117 there is shown in broken lines the input potential of the modulation amplifier, coinciding with Fig. 1. The solid curve in Figure 11) represents the artificially distorted output potential of the modulation amplifier M as supplied to the transmitter input. To compensate the fatigue effect the modulating potential as shown in Fig. 1a, therefore, must obviously increase over a period of time if the signal endures for a long period. The effect of an arrangement of this nature is equivalent to a lifting of the lowest frequencies of the signal.

According to the invention, therefore, the modulation amplifier is provided with a lifting of the low tones, i. e., an ascending frequency curve in the case of frequencies of the order of the mains periods. A complete compensation of the error in the transmission stage takes place when the chronological lift of the distorted modulating potential curve is exactly identical with the chronological drop in the transmitter energy. For this purpose certain conditions require to be fulfilled as regards the time constant of the low-tone increase in relation to the time constant of the plate potential storage means in the transmitter. The plotting of these relations and the dimensioning of the correcting circuits are preferably performed oscillographically. Of the numerous possible circuit systems with which the idea according to the invention is carried out in practice, merely one, which has been found to be particularly suitable, is shown in Fig, 2. From the image amplifier l9 there is obtained an ideal, i. e. undistorted wave according to Fig. 1. This passes to a modulation amplifier M. In order to obtain the output potential behind M with the same polarity as the input potential the modulation amplifier has two stages 20 and 20'. The two tubes 20 and 28' have the operating re sistance 2| and 2| For lifting the low tones in accordance with the invention there are employed the circuits 22, 23 and 22, 23' the time constants of which are just balanced in such fashion that the fatigue error of the transmitter is just compensated. There is coupled with the transmitter the total potential of the high-frequency resistance 2! and the correcting circuit 22, 23. This total potential is chronologically distorted and is of the form shown in Fig. 1b. The correction may naturally also be performed in one stage. When performed in a plurality of stages one effects a correction of the modulation potential in a higher degree. The transmission tube 30, as regards bias, is coupled galvanically with the output tube 20' of the modulation amplifier arrangement M. It is excited from the source 3| and is supplied with plate potential by the storage condenser ll. To safeguard against failure on the part of the grid bias, for example owing to a defect of the output tube 26', there is employed the grid bias resistance 24, which is also bridged by an electrolytic condenser 25 which has low impedance for even the lowest image frequencies. The time constant of the anode storage means for the discharge is then governed by the internal resistance of the tube 39 and the capacity l1, whilst in respect of charging it is governed in substance by the rec tifying tube It in conjunction with I! and the primary condenser 18. The low-tone increase on the part of the correcting means in M must then be tuned individually in each case to these particular time constants of the transmitter rectifier.

In the circuit diagram of the modulation amplifier arrangement M there is also shown a reaction coupling 26, 21, which acts by way of a rectifier 28 on the grid. This reaction coupling then maintains the basic value of the control potential at the same level, and in this Way it is effected that the modulation impulses are based always on a definite basic potential value. In addition, attention is directed to the grid bias 29. If this bias is increased from zero into negative, the peak value of the image is varied in the case of positive transmission. In this way the ratio between synchronisation modulation and image modulation is adjustably varied in the case of continuously constant difference in energy between idle operation and top load, which is of great importance as regards practical transmission.

The stated considerations according to the invention apply to negative transmission after corresponding variation of the error sign. In negative transmission the fatigue error, which is then in the opposite direction, can also be eliminated in practice according to the load compensating method, the screening grid method and, with particular success, the low-tone correction.

Suitable values for the components shown in Fig. 2 are:

2i and 2 I '=500 ohms, 22 and 22'=500 ohms, 23 and 23'=16 mf.

I claim:

A system for a television transmitter for compensating for variations in modulator power supply potential comprising a source of modulation potential, an amplifier tube having a cathode, a control electrode and an anode, means for applying the modulation potential between the cathode and control electrode of said amplifier tube, a source of energizing potential for said amplifier tube, means including a pair of series connected load resistances and said source of energizing potential for maintaining said anode positive with respect to said cathode, a condenser connected between the junction of said load resistances and a point of fixed potential, at modulator tube including a cathode, a control electrode and an anode, means for coupling the anode of said amplifier tube to the control electrode of said modulator tube, means for impressing voltage variations of carrier frequency upon the control electrode of said modulator tube, an anode power supply for said modulator tube including a rectifier and a filter network, said filter including a series inductance and parallel capacitance, the terminal voltage of said modulator anode power supply being subject to variations with current demand of said modulator tube, the value of said first-named condenser and its time constant with respect to one of the said load resistances being so chosen as to substantially fully compensate for said variations in the modulator anode power supply voltage at said filter terminals by reason of variations in the current de mand of said modulator tube.

KURT SCHLESINGER. 

